Timely and accurate monitoring of droughts is important for implementing an effective response and for minimizing economic losses. In this paper, the duration and amount of deficit in precipitation before every severe drought were analyzed for 124 meteorological stations based on the weather data from 1961 to 2012. The results showed that deficit in precipitation over a period as short as three months or even shorter could lead to severe drought and the cumulative timescales of precipitation deficit in southwestern China were longer than those in southern China. The distribution of the critical amount of precipitation deficit showed a clear regional difference. Deficits in the western parts of southwestern China and parts of southern China are above 60%, or even above 80%, higher than the other area ranging between 40% and 60%. On the whole, the critical amount of the deficit preceding severe droughts for the humid southwestern and southern China was lower than that for the semiarid and semihumid areas. The results offer a sound basis for monitoring and forecasting droughts in southern and southwestern China and for issuing early warnings.
Drought is the world’s costliest stochastic natural disaster affecting more people than any other natural disaster [
Timely and accurate monitoring of droughts is important for implementing an effective response and for minimizing economic losses [
Drought is essentially a shortage of water caused by an imbalance in the supply of and the demand for water, usually triggered by a severe and persistent deficit in precipitation [
Based on the above analysis, those areas in southwestern and southern China that had faced severe or extreme droughts—as judged by the
The study areas in southwestern (refer to Sichuan, Chongqing, Yunnan, and Guizhou provinces) and southern China (refer to Guangxi and Guangdong provinces) comprise 124 meteorological stations. Nearly all stations experience a subtropical climate; however, northwestern Sichuan and the northernmost part of Yunnan are in the plateau climate zone and the southern parts of Yunnan and Guangdong provinces experience a tropical climate. The elevation ranges from below 400 m in the southeast to more than 3000 m in the northwest (Figure
The location of southwestern and southern areas in China, the spatial distribution of elevation (unit: m), and 124 meteorological stations (dot) in the study area.
For each station, data spanning the years 1961–2012 were collected for a number of variables including monthly mean temperature, maximum temperature, minimum temperature, precipitation, sunshine hours, average wind speed, and relative humidity. All the data were obtained from the National Meteorological Information Center and meet the stipulated criteria for quality control.
The degree of drought severity is expressed with
The
Following the approach proposed by Svoboda et al. [
Categories of the severity of drought as indicated by the
Category | Degree of severity | Percentile chance (%) |
---|---|---|
1 | No drought | >30 |
2 | Incipient | 15 to ≤30 |
3 | Moderate | 5 to ≤15 |
4 | Severe | 2 to ≤5 |
5 | Extreme | ≤2 |
The methods for calculating the cumulative timescales of precipitation deficit and the critical amounts of the cumulative precipitation deficit for each station are as follows. First, the severity of drought at each station is determined according to the monthly
The
As ascertained from the related literatures [
The results from monthly monitoring of the
Distribution of droughts of varying severity as judged by the
Figure
Frequency distribution of cumulative timescales.
Figure
The first 4 cumulative timescales of precipitation deficit (red markers map, unit: month: (a) the first; (b) the second; (c) the third; (d) the fourth) and the proportion of droughts (blank map, unit: %) due to the deficits of each timescale.
The second timescale was basically 2 months and accounts for 20%–30% of the total in most part of Sichuan, the western and southeast parts of Guizhou, and the western part of Guangxi and accounts for 10%–20% in the other areas (Figure
The third timescale was mainly 3 months, which accounts for 10%–20% of severe droughts (Figure
The fourth timescale was 4–6 months (Figure
It can be seen that those severe droughts due to extended periods (4–6 months or longer) of precipitation deficit were fewer: most of them are accounted for by deficits lasting up to 3 months. However, droughts due to extended periods of deficit are usually rare, perhaps once in several decades or a century. For example, the severe summer drought in Sichuan and Chongqing in 2006 was preceded by a deficit of 4-5 months in most of the areas (Figure
Cumulative months of negative precipitation accumulation: June–August 2006.
Cumulative months of negative precipitation accumulation: October 2009–March 2010.
The cumulative timescales of precipitation deficit that preceded severe droughts in southwestern China were longer than those in southern China. Severe droughts in southern China were caused mainly by 1-month deficits, whereas in most parts of Sichuan and northern Yunnan, besides the droughts due to 1-month deficit, those due to deficits longer than 2 months are more frequent than other areas.
The analysis in Section
Figure
The first 4 precipitation deficits (colored dot, unit: %: (a) the first; (b) the second; (c) the third; (d) the fourth) under one-month timescale and the proportion of droughts (blank map, unit: %) due to varying degrees of deficit.
As the droughts caused by precipitation deficits around 40%–100% accounted for about 90% of the total number of severe droughts, each of the amounts of deficit was given a weighting equivalent to the percentage of the total number of droughts due to that category of deficit (Figure
Weighting precipitation deficit (colored dot, unit: %) and the percentage of droughts accounted for by each deficit (blank map, unit: %). The timescale is 1 month.
Figure
Weighting precipitation deficit (colored dot, unit: %) and the percentage of droughts accounted for by each deficit (blank map, unit: %). The timescale is 2 months (a) and 3 months (b).
To examine regional differences in the critical threshold of precipitation deficit in greater detail, the research area was divided into three subareas, namely, the western parts of southwestern China (Yunnan and Sichuan), the eastern parts of southwestern China (Guizhou and Chongqing), and southern China. The division was based on the characteristics of the spatial distribution at different timescales and under varying amounts of the deficit discussed above. The relationship between the cumulative timescales and the cumulative amount of precipitation deficit was examined by plotting their respective values for each of the three subareas. Another area—the semiarid and semihumid areas—was also introduced in the same graph (Figure
Relationship between cumulative timescales and cumulative precipitation deficit in different regions.
(1) Most of the severe droughts in the study area were caused by a precipitation deficit of less than 3 months. The timescale was shorter—of 1 month—in southern China, the southern parts of Yunnan and Chongqing, than that in the remaining areas. However, besides those due to a 1-month timescale, severe droughts due to a deficit of more than 2 months were more frequent in the northern parts of southwestern China than in the remaining areas.
(2) The distribution of the critical values of precipitation deficit showed clear regional differences, especially on the timescales of 1-2 months. Severe droughts occurred in the eastern parts of southwestern China and in north-central Guangxi if the monthly precipitation deficit was 60% and even 40% in some small pockets. In other areas, the critical amount of deficit was more than 80%. When the duration was 2 months, the critical value of the amount of deficit was 60%–80% in the western parts of southwestern China and the coastal areas of southern China and 40%–60% in most of the remaining areas. When the duration was 3 months, the critical value was mainly 40%–60%. When the duration was more than 3 months, the critical value was generally as low as about 40%. Overall, the critical value of the amount of precipitation deficit for the eastern parts of southwestern China and most of the areas in southern China was lower than that for the western parts of southwestern China. The longer the timescale, the lower the critical value and the smaller the regional differences.
(3) Regional differences in the critical values of the length of the deficit and its amount were related to the differences in geographic position and climate. The western parts of southwestern China, especially the West Sichuan Plateau and north-central Yunnan, are at a higher elevation and the average temperatures at these higher altitudes are 10–12°C lower and evaporation is also 200–400 mm lower. Therefore, even in the absence of any precipitation deficit, the greater evaporation caused by the higher temperatures can accelerate the development of drought in southern China and in the eastern parts of southwestern China, which is why even small amounts of precipitation deficit in these areas can cause severe droughts in a relatively short time. Zhang et al. [
The authors declare that they have no competing interests.
The authors are grateful for the National Meteorological Information Center of CMA for providing the data used in this study. This research was supported by the National (Key) Basic Research and Development (973) Program of China (Grant no. 2013CB430200).